Slice onions thinly and soften in butter over low heat. Increase heat and begin to colour the onions. Add sugar and vinegar and continue to cook until the onions take on significant colour and there is no liquid left.

Slice 5 1/3oz cheese into small pieces and lay a piece in each tart. Add a small amount of onions followed by one piece of sun-dried tomato.

Bring Béchamel to boil, remove from heat and whisk in egg yolks very quickly. Add remaining cheese; grated or chopped. Whisk egg whites until stiff peaks form. Mix 1/3 of egg whites to the Béchamel and cheese sauce. Carefully fold in the remaining egg white until a consistent mix is obtained. (Be careful not to be too heavy-handed as all the air will be released from the egg whites.) Spoon a small amount on top of each tart and cook for 5 minutes in pre-heated oven at 410 degrees F. Serve immediately.